Niese gave up one run and two hits in five innings Sunday in the Mets' 2-1 loss to the Atlanta Braves.
With Santana still building up arm strength -- he hasn't pitched in a single spring training game -- Niese appears to be the likely replacement for the April 1 opener against San Diego.
"I'm not convinced, but there's certainly a strong possibility that's going to happen," Mets manager Terry Collins said.
With both clubs sporting some green in their uniforms and playing with green bases on St. Patrick's Day, Niese struck out four and walked one. He allowed Chris Johnson's home run in the fourth, Johnson's third of spring training. The drive went off the scoreboard in left-field.
"It was one of those things," Niese said. "I got two quick outs and should've been pounding the zone there.
I nibbled with my changeup and fell behind."
Atlanta worked the count and ran Niese's pitch count up to 83.
Valdespin's homer landed on the walkway beside the berm down the right-field line, his fourth home run of the exhibition season.
"He hit a four-seam fastball," said Teheran. "I was behind in the count, so I tried to get ahead. He was jumping on me. I just tried to stay focused on the game and compete after that."
Teheran lowered his ERA to 1.35.
"I've been trying to make the team and trying to impress them," he said. "That's why I've been doing good. A lot of things are working better. I'm throwing my breaking pitch more for strikes. That's the difference from last year. I tried to throw everything by everybody last spring."
He received some advice during the offseason from a pitcher he idolized, former-Met Pedro Martinez. The two had dinner together during winter ball in the Dominican Republic.
"He told me to save my energy for the whole game, and that's what I'm trying to do this year," Teheran said. "It was exciting for me because when I was little, I watched him pitch."
Freddie Freeman hit an RBI single in the sixth off Scott Rice. Johnson had three hits.
Atlanta gave its regular outfield (Jason Heyward, Justin Upton and B.J. Upton) a day off. ... Mets INF Justin Turner, who sprained his right ankle while fielding a grounder at third base on Saturday, had X-rays, which were negative. ... Collins described 2B Daniel Murphy as "pretty stiff" and said the team will see how he is on Wednesday. Murphy has not appeared in a game yet because of a strained right intercostal muscle. ... Collins also said there is no schedule for Santana to pitch a game this week.